In the LabChart Spirometry module, what is the definition for the Dead Air Space (DAS) volume correction factor?

The dead space air (DAS) volume is the total volume of air in the tubing of the apparatus; plus, the air that is not normally heated by the body (air in mouth, throat, etc.). 
For humans, a DAS value of 400mL is typical. In canines, 65-75mL wouuld be more appropriate.